Chlorophyll as a benchmark for vegetable quality
Without blanching, the enzymes catalase and peroxidase will accelerate
changes in texture, color, and flavor even after the food has been frozen.
There are several ways to follow changes in vegetables after processing,
including measuring ascorbic acid or, in the case of cauliflower, the
brown pigments soluble in a water-acetone mixture. Many T-TT studies,
however, followed the fate of chlorophyll.
The chlorophyll content of a living plant is readily measured. The blanching
procedure ends the respiration process, and the conversion of chlorophyll
to pheophytin begins. At the same time, the color slowly changes from
a bright green to a dull olive brown. With proper handling and processing,
it can be assumed that at the beginning, when the food is freshly frozen,
there is no pheophytin present, and any changes in the ratio of chlorophyll
to pheophytin are taking place in the post-freezing chain of events from
processor to consumer. This proved to be a sensitive technique for the
WRRC to follow what was happening as time and temperature fluctuated.
It was actually possible to construct empirical equations that would predict
the chlorophyll conversion based on the temperature history of a frozen
The T-TT investigators found that "the first perceptible loss of
initial quality of many frozen vegetables occurs in less than one years
storage at 0° F." Further, "a few degrees difference in
temperature for long-term storage can cause a great difference in quality
retention. At the same time, short exposures above 0° F are no more
damaging than long exposures at lower temperatures." (5)
These results testify to the complexity of the frozen-food stability problem.
For example, on storage at 0° F, the first noticeable flavor difference
occurred first for peas (265 days), followed by spinach (271 days), green
beans (285 days), and cauliflower (355 days).